ANS:B - Sulphuric acid

To test the chemical oxygen demand (C.O.D.) of sewage, organic matter is oxidized by potassium dichromate (K2Cr2O7) in the presence of sulfuric acid (H2SO4). The role of sulfuric acid in this test is to provide the acidic conditions necessary for the oxidation reaction to occur effectively. Sulfuric acid serves as a catalyst for the reaction and helps to maintain the acidic pH required for the oxidation of organic matter by potassium dichromate. Therefore, the correct answer is: Sulphuric acid.